For CosmoMC, I have set the num_threads such as 8, 24, etc. and it ran well at beginning. However, when it had been going to converge down, for example, R-1 from about 50 to 10 or less, sometimes it would run only with one thread rather than what I had set. But if R-1 remained around a large number and did not go down, it would not happen. Even if I stopped running and restarted the chains with checkpoints, it would happen again.

Since it ran with one thread, it was quite inefficient. I have no idea to deal with the problem.